Question: In geometry class, the girl to boy ratio is $10$ to $5$. If there are $20$ boys, how many girls are there?
Solution: A ratio of $10$ girls to $5$ boys means that there is a group of $10$ girls for every group of $5$ boys. If there are $20$ boys, then there are $4$ groups of $5$ boys each. Because we know the ratio is $10:5$ , there must be $4$ groups of $10$ girls each. There is a total of $40$ girls in geometry class.